Automatic material discrimination by using computer tomography

ABSTRACT

Method and apparatus are provided for combining information obtained from CT and Coherent Scatter Computed Tomography to better determine whether there are dangerous materials in the baggage or not. Hence, the attenuation coefficient and the diffraction pattern of the item of baggage are used to determine whether the baggage should be cleared.

The present invention relates to the field of material discrimination.In particular, the present invention relates to an inspection system fordetecting a specific material of interest in an object such as an itemof baggage, a method of inspecting an object such as an item of baggageand to a computer program stored on a computer readable medium.

Over the past several years, x-ray baggage inspections have evolved fromsimple x-ray imaging systems that were completely dependent oninteraction by an operator to more sophisticated automatic systems thatcan automatically recognize certain types of contrabands and trigger alldangerous materials. The newer inspection systems have employed singleenergy or dual energy x-ray radiation transmitted through or scatteredfrom the examined package. Some systems have used a single view sourcedetector arrangement, others have utilized a dual view or multi-viewarrangements. The single or dual view systems usually scan baggage, asit moves on a conveyer, using a scan beam or scanning pencil beam ofx-rays in a fixed geometry. The multi-view, computer tomography (CT)type systems usually scan stationary baggage in a fixed geometry of scanangles and process data corresponding to absorption of x-rays toreconstruct selected slices of the baggage. Known CT-scanners also applyhelical scanning mode, thus producing 3D images of the attenuationcoefficient of the object.

At airports, the baggage inspection procedure is usually divided in toat least 2 levels of inspection. A first level system processes baggagerapidly, up to a rate of 1500 bags per hour. This first level system islocated at a first inspection station and inspects all baggage. Thesystem rapidly scans baggage and automatically makes a decision based onits particular modes of detection and methodology. Usually, CT scannersor dual energy transmission X-ray are used as first level systems, whichdetermines the attenuation coefficient of the bag or of an area of thebag. Subsequently, the attenuation coefficient is compared to theattenuation coefficient of dangerous materials. In case the attenuationcoefficient of the scanned item of baggage matches the known attenuationcoefficient of a dangerous material, an alarm is issued or the item ofbaggage is separated from the main stream of baggage for furtherinspection.

In case there is a group of materials consisting of non-dangerousmaterials and dangerous materials having an attenuation coefficient thatmatches the attenuation coefficient of the item of baggage, the item ofbaggage is forwarded to the second level. At the second level anoperator usually visually inspects an x-ray image of the rejected itemof baggage and attempts to determine whether a suspicious object insidethe item of baggage can be cleared based on its obvious shape. Theoperator searches the image for characteristic objects such as weapons,timing and detonation devices, wires or other characteristics associatedwith the contraband. In case the operator cannot clear the item ofbaggage, vapor or trace detectors or further CT scanners may be used tofurther inspect the item of baggage.

Reference EP 127 546 A2 discloses a computer tomograph using primaryradiation as well as diffraction radiation for determining anexamination result.

Reference U.S. Pat. No. 5,642,393 discloses an inspection systemcomprising a multi-view x-ray inspection probe constructed to employx-ray radiation transmitted through or scattered from an examining itemto identify a suspicious region inside the examined item. The multi-viewx-ray inspection probe is constructed to identify the suspicious regionusing several examination angles of the transmitted or scattered x-rayradiation. Furthermore, the multi-view x-ray inspection probe isconstructed to obtain spatial information of the suspicious region todetermine a geometry for subsequent examination. Furthermore, adirectional, material sensitive probe is provided, constructed toacquire material specific information about the suspicious region byemploying the geometry. On the basis of the material specificinformation, a presence of a specific material in the specific region isdetermined.

It is an object of the present invention to provide for an unambiguousautomatic material discrimination.

According to an exemplary embodiment of the present invention, the aboveobject may be achieved with an inspection system for detecting aspecific material of interest in an item of baggage, with the inspectionsystem comprising a first scanner system for determining an attenuationcoefficient of the item of baggage, a second scanner system fordetermining a diffraction pattern of the item of baggage and acalculation unit connected to the first and second scanner systems foridentifying a presence of the specific material of interest in the itemof baggage on the basis of the attenuation coefficients and thediffraction pattern. Advantageously according to this exemplaryembodiment of the present invention, a very reliable identification ofthe specific material of interest can be provided. Also, since theattenuation coefficient of the item of baggage can be determined veryrapidly, a two step process can be realized with the inspection systemaccording to this exemplary embodiment of the present invention by usingthe first scanner system as the first level system screening the flow ofbaggage for “suspicious bags” where the “suspicious” bags are thenfurther inspected by using the diffraction pattern of the item ofbaggage. Advantageously, since the diffraction pattern allows for a veryreliable determination of the material of interest, a fault rate of theinspection system according to this exemplary embodiment of the presentinvention is reduced significantly.

According to the exemplary embodiment of present invention, the firstand second scanner systems are arranged at a distance from each other,allowing to provide for example a conveyer belt switch between the firstand second scanner systems such that a “suspicious” item of baggage mayeasily be branched off and does not have to pass the second scannersystem. For example, a plurality of first scanner systems may beconnected by conveyer belts and respective conveyer belt switches to thesecond system to thereby insure a high utilization ratio of the secondscanner system.

According to another exemplary embodiment of the present invention, thefirst scanner system is a CT scanner system and the second scannersystem is a coherent-scatter CT system. Advantageously, this allows theuse of known first level CT scanners in the inspection system accordingto the present invention, in combination with a coherent-scatter CTsystem (CSCT).

According to another exemplary embodiment of the present invention, thefirst and the second scanner systems are realized as one scanner systemhaving one source of radiation and one detector system. For implementingthe CT scanner for determining the attenuation coefficient of the itemof baggage, a first aperture system is provided between the source ofradiation and the item to be scanned. To implement the CSCT scanner fordetermining the diffraction pattern of the item of baggage, a secondaperture system such as a slot aperture or diaphragm is provided to formthe radiation of the one source of radiation into a fan beam.Advantageously, this exemplary embodiment of the present invention hascompact dimensions and may easily be installed in airport securitysystems where space is very often a problem. Furthermore, since only onesource of radiation and one detector system is necessary, the costs formanufacturing such a system can be provided as well as the amount ofmoving parts underlying wear and tear can be reduced.

According to another exemplary embodiment of the present invention, amethod of inspecting an item of baggage includes scanning the item ofbaggage at a first scanner stage for determining an attenuationcoefficient of the item of baggage, determining whether there is asuspicious region in the item of baggage on the basis of the attenuationcoefficient, scanning an area of the item of baggage including thesuspicious region at a second scanner stage for determining adiffraction pattern of the area and determining whether there isdangerous material in the item of baggage on the basis of thediffraction pattern. Advantageously, this exemplary embodiment of thepresent invention allows for a two step inspection process allowing tomake the inspection process very efficient and dependable. The use ofthe diffraction pattern to determine whether there is dangerous materialor not allows for a very low fault rate of the method.

Further exemplary embodiments according to the present invention providefor a fast and efficient inspection of items of baggage while minimizingcomputation efforts necessary when these methods are applied oncomputerized inspection systems. In particular, the issuance of aplurality of alarms with respect to whether a dangerous material hasbeen identified in the item of baggage or whether there is material inan item of baggage which cannot be identified allows an operator of theinspection system to easily identify the item of baggage in question andto identify the reason for the alarm.

Another embodiment relates to a computer program stored on a computerreadable medium which executes the steps of the method according to thepresent invention when executed on an inspection system. Advantageously,this computer program allows for a reduction of computation power in theinspection system.

It may be seen to be the gist of an exemplary embodiment of the presentinvention that a conventional CT-system is used for determining theattenuation coefficient in a first stage to identify the material underinvestigation. However, in case the result is ambiguous, informationobtained from a CT-system and from a CSCT-system is combined to betterdiscriminate materials. In medical applications this procedure can beused to distinguish between healthy and non-healthy tissue. In baggageinspection applications, materials with similar attenuation can bedistinguished resulting in a lower false alarm rate of the system andtherefore in a higher degree of automatization.

These and other aspects of the present invention will become apparentfrom and elucidated with reference to the embodiments describedhereinafter.

Exemplary embodiments of the present invention will be described in thefollowing with reference to the following drawings:

FIG. 1 shows a schematic representation of an exemplary embodiment of acomputer tomograph according to the present invention as it may be usedfor baggage inspection.

FIGS. 2 a, 2 b show a flow-chart of an exemplary embodiment of a methodfor operating the computer tomograph of FIG. 1 or the computer tomographof FIG. 3.

FIG. 3 shows a schematic representation of another exemplary embodimentof a computer tomograph according to the present invention.

FIG. 1 shows a schematic representation of an exemplary embodiment of acomputer tomograph according to the present invention. The computertomograph depicted in FIG. 1 comprises a gantry 1, which is rotatablearound a rotation axis 2. The gantry 1 is driven by means of a motor 3.Reference character 4 designates a source of radiation such as an x-raysource.

Reference character 5 designates a first aperture system which forms theradiation beam emitted from the radiation source 4 to a cone shapedradiation beam 6 passing through the item of baggage 7. After passingthrough the item of baggage 7, the cone beam 6 impinges onto a detectorarray 8. The aperture system 5 is arranged such that the cone beam 6covers the whole of the detector 8.

Furthermore, there is provided another aperture system 9 consisting of adiaphragm. The diaphragm has the form of a slit 10 such that theradiation emitted from the source of radiation 4 is formed into a fanbeam 11.

The fan beam 11 and the cone beam 6 pass through the item of baggage 7arranged in the center of the gantry 1 and impinge on to the detector 8.As shown in FIG. 1, the detector is attached to the gantry 1 opposite tothe radiation source 4. The detector 8 consists of a two-dimensionaldetector array comprising a plurality of elements arranged in the formof a matrix. The individual detector elements are arranged in lines andcolumns. The columns are parallel to the rotation axis 2 whereas thelines are arranged in planes perpendicular to the rotation axis 2.

The apertures of the aperture systems 5 and 9 are adapted to thedimensions of the detector 8 such that the scanned area of the item ofbaggage 7 is within the cone beam 6 or the fan beam 11 and that thedetector 8 covers the complete scanning area. As may be taken from FIG.1, preferably the slit 10 of the aperture system 9 is arranged such thatthe fan beam 11 is mapped on the middle line 15 of the detector 8.

During a scan of the item of baggage 7, the radiation source 4, theaperture systems 5 and 9 and the detector 8 are rotated along the gantry1 in the direction indicated with arrow 16. For rotation of the gantry 1with the source of radiation 4, the aperture systems 5 and 9 and thedetector 8, the motor 3 is connected to a motor control unit 17 which isconnected to a calculation unit 18.

In FIG. 1, the item of baggage 7 is disposed on a conveyer belt 19.During the scan of the item of baggage 7, while the gantry 1 rotatesaround the item of baggage 7, the conveyer belt 19 displaces the item ofbaggage 7 along a direction parallel to the rotation axis 2 of thegantry. By this, the item of baggage 7 is scanned along a helix. Theconveyor belt can also be stopped during the scan thus measuring singleslices.

The detector 8 is connected to the calculation unit 18. The calculationunit 18 receives the detection results from the detector 8 anddetermines a scanning result on the basis of the detection results fromthe detector 8. In addition to that, the calculation unit 18communicates with the motor control unit 17 in order to coordinate themovement of the gantry 1 with the motors 3 and 20 or with the conveyerbelt 19. Furthermore, there is provided a loud speaker 21 connected tothe calculation unit 18 for issuing an alarm in case the calculationunit determines that there is a dangerous material within the item ofbaggage 7 or a material which cannot be determined. A data port 22 cantransport the alarm signal to a subsequent detection level.

As may be seen from FIG. 1, depending on which of the aperture systems 5and 9 is in use, the computer tomograph of FIG. 1 can either be aconventional scanner or a CSCT-scanner: in case the aperture system 5 isactive, the computer tomograph is a CT-scanner and in case the aperturesystem 9 is active, the computer tomograph is a CSCT-scanner.

FIGS. 2 a and 2 b are a flow-chart of an exemplary embodiment of amethod for operating the computer tomograph of FIG. 1. After the startin S1, the method continues to step S2 in which the item of baggage 7 istransported to the first scanner stage by means of the conveyer belt 19.Then, the method continues to step S3 in which the item of baggage 7 isscanned at the first scanner stage to determine the attenuationcoefficient of the baggage. In detail, during the scan of the item ofbaggage 7 at the first scanner stage, the radiation source 4 and thedetector 8 on the gantry 1 are rotated around the item of baggage 7.During this, the conveyer belt 19 moves the item of baggage 7 throughthe scan area covered by the cone beam 6 emitted by the radiation source4 with the aperture system 5 such that the item of baggage 7 is scannedin its entire length. Due to the movement of the item of baggage 7 onthe conveyer belt 19 and the rotation of the gantry, the item of baggage7 is scanned along a scanning helix. The detection results of thedetector 8 are transmitted to the calculation unit. Then, the methodcontinues to step S4 where the calculation unit 18 determines whetherthe attenuation coefficient of the item of baggage 7 determined fromdetection results of the detector 8 corresponds to the attenuationcoefficient of a known dangerous material. For this, the determinedattenuation coefficient of the item of baggage 7 is compared to a tableof attenuation coefficients of known dangerous materials. In case it isdetermined in step S4 that the attenuation coefficient of the baggage 7corresponds to the attenuation coefficient of a known dangerousmaterial, the method continues to step S7 where the calculation unit 18issues a first alarm by means of the loudspeaker 21 indicating that theitem of baggage 7 contains dangerous material. Then, from step S7 themethod continues to step S8 as indicated by the encircled A at thebottom of FIG. 2 a and at the top of FIG. 2 b.

At step S8, the item of baggage 7 is transported to a location where amanual inspection or a different subsequent threat detection method iscarried out. From step S8 the method continues to step S22 where itends.

In case it is determined in step S4 that the attenuation coefficient ofthe item of baggage 7 does not correspond to the attenuation coefficientof a dangerous material, the method continues to step S9 where thecalculation unit 18 determines whether the attenuation coefficient ofthe item of baggage 7 corresponds to the attenuation coefficient of agroup of materials consisting of dangerous and non-dangerous materials.In other words, in step S9 it is determined whether there is asuspicious region which may contain dangerous material in the item ofbaggage 7. In case it is determined in step S9 that there is nosuspicious region, i.e. that the attenuation coefficient of the item ofbaggage 7 does not correspond to the attenuation coefficient of a groupof materials consisting of dangerous and non-dangerous materials, themethod continues to step S10 where the baggage is transported to itsdestination by means of the conveyer belt 19. Then, after step S10, themethod ends at step S22.

In case it was determined in step S9 that there is a suspicious regionin the item of baggage 7, the method continues to step S11 where theitem of baggage 7 is transported to the second scanner stage. In thecomputer tomograph of FIG. 1, the transportation step 11 is carried outsuch that the conveyer belt either changes its direction such that theitem of baggage 7 it moved through the scanning area of the computertomograph in a backward direction during the scan at the second scannerstage or the item of baggage 7 is returned to its initial positionbefore the scan at the first scanner stage and then for the scan at thesecond scanner stage, the item of baggage 7 is again moved along adirection parallel to the rotation axis of the gantry 1 during the scanat the second scanner stage.

From step S11, the method continues to step S12 where an area isdetermined which is to be scanned at the second scanner stage. The areaincludes the suspicious region determined in step S9. Depending onwhether the item of baggage was moved on the conveyer belt 19 or wasdeformed since the scan at the first scanner stage, the area to bescanned at the second scanner stage can be enlarged or can be limited tothe suspicious region determined in step S9. Step S12 is carried out bymeans of the calculation unit 18. Then, as indicated by means of theencircled B at the bottom of FIG. 2 a and the encircled B at the top ofFIG. 2 b, the method continues to step S13 where the area of the baggagedetermined in step S12 is scanned at the second scanner stage todetermine the diffraction pattern of the area of the baggage. For this,as already indicated above, the item of baggage 7 is transported along adirection parallel to the rotation axis 2 of the gantry 1. When thesuspicious region is penetrated by the fan-beam, the belt is stopped andthe radiation source 4, the aperture system 9 and the detector system 8are rotated around the item of baggage 7 by means of the gantry 1. Fordetermining the diffraction pattern of the area of the item of baggage7, only the scattered radiation is used by the calculation unit 18.Then, after the determination of the diffraction pattern in step S13,the method continues to step S14 where the diffraction pattern ismatched to known diffraction patterns of known materials. For matchingthe diffraction pattern to known diffraction patterns of knownmaterials, the diffraction pattern of the area of the item of baggage 7is compared to a table consisting of known diffraction patterns of knownmaterials.

After step S14, the method continues to step S15 where a query is madewhether the diffraction pattern of the area of the baggage matches theknown diffraction pattern of a known dangerous material. In case thediffraction pattern of the area of the baggage can be linked to adangerous material, the method continues to step S16 where thecalculation unit which performed steps S14 and S15 issues a secondalarm. From step S16 the method continues to step S8.

In case it was determined in step S15 that the diffraction pattern ofthe area of the baggage 7 does not match a known dangerous material, themethod continues to step S17.

In step S17 the calculation unit makes a query whether the diffractionpattern corresponds to the known diffraction pattern of a group ofmaterials consisting of dangerous and non-dangerous materials. In caseit is determined in step S17 that the diffraction pattern of the item ofbaggage 7 can be linked to a group of materials consisting of dangerousand non-dangerous materials the method continues to step S18 where thecalculation unit issues a third alarm by means of the loudspeaker 21.From step S18, the method continues to step S8.

In case it was determined in step S17 that the diffraction pattern doesnot correspond to the diffraction pattern of a group of materialsconsisting of dangerous and non-dangerous materials, the methodcontinues to step S19 where a query is made whether the diffractionpattern can be matched to a known diffraction pattern of a knownnon-dangerous material. In case it is determined in step S19 that thediffraction pattern of the area of the item of baggage 7 cannot belinked to a non-dangerous material the method continues to step S20where the calculation unit 18 issues a fourth alarm by means of theloudspeaker 21. From step S20 the method continues to step S8.

In case the calculation unit 18 determines in step S19 that thediffraction pattern of the item of baggage 7 can be linked to a knownnon-dangerous material, the method continues to step S21 where the itemof baggage 7 is transported to its destination by means of the conveyerbelt 19. Then, from step S21 the method continues to step S22 where itends.

In a variant of the method depicted in FIGS. 2 a and 2 b, in addition tothe diffraction pattern, the calculation unit 18 may also use theattenuation coefficient to determine in steps S15, S17 and S19 whetherthe material included in the item of baggage 7 includes dangerousmaterial or not.

FIG. 3 shows another exemplary embodiment of the computer tomographaccording to the present invention. Reference character 30 depicts afirst scanner stage comprising a CT-scanner. Reference character 31designates a second scanner stage comprising a CSCT-scanner. Each of theCT-scanner and the CSCT-scanner comprises a radiation source and adetector system as described with reference to FIG. 1. The first scannerstage 30 and the second scanner stage 31 are connected to each other bymeans of a conveyer belt 32 for transporting an item of baggage 33 fromthe first scanner stage 30 to the second scanner stage 31.

The first scanner stage 30 and the second scanner stage 31 are connectedto a calculation unit 34 which is connected to a loudspeaker 35. Theitem of baggage 33 to be inspected is firstly scanned at the firstscanner stage 30. At the first scanner stage 30, the attenuationcoefficient of the item of baggage 33 is determined and transmitted tothe calculation unit 34. At the calculation unit 34, the attenuationcoefficient determined at the first scanner stage 30 is compared to atable of known attenuation coefficients of known materials. In case theattenuation coefficient of the item of baggage 33 can be matched to theknown attenuation coefficient of a dangerous material, the calculationunit issues a first alarm by means of the loudspeaker 35.

In case the attenuation coefficient of the item of baggage 33 can bematched to an attenuation coefficient of a non-dangerous material, thecalculation unit controls the operation of the conveyer belt 32 suchthat the item of baggage 33 is transported to its destination. In casethe item of baggage 33 passes the second scanner stage 31 on its path toits destination, the calculation unit 34 controls the second scannerstage 31 such that no scan is performed at the second scanner stage 31if it was determined that there is only non-dangerous material in theitem of baggage 33.

In case the calculation unit 34 matches the attenuation coefficient ofthe item of baggage 33 to the known attenuation coefficient of a groupof materials consisting of dangerous and non-dangerous materials, thecalculation unit 34 controls the conveyer belt 32 such that the item ofbaggage 33 is transported to the second scanner stage 31 where the itemof baggage 33 is scanned by means of the CSCT-scanner. On the basis ofthe scanning results, a diffraction pattern is determined at: the secondscanner stage 31, which is transmitted to the calculation unit 34. Then,the calculation unit 34 compares the diffraction pattern of the item ofbaggage 33 to known diffraction patterns of known materials. In case thediffraction pattern of the item of baggage can be linked to dangerousmaterials, the calculation unit 34 issues a further alarm by means ofthe loudspeaker 35. In case the calculation unit 34 links thediffraction pattern of the item of baggage 33 to non-dangerousmaterials, the calculation unit 34 controls the conveyer belt 32 suchthat the item of baggage 33 is transported to its destination.

In case the diffraction pattern of the item of baggage 33 is linked to agroup of materials consisting of dangerous and non-dangerous materials,the calculation unit 34 issues a further alarm by means of theloudspeaker 35 and controls the conveyer belt 32 such that the item ofbaggage 33 is transported to a further point of inspection where theitem of baggage 33 is inspected by a person.

Despite of the fact that the above method and apparatus are describedwith respect to baggage inspection, the above method and apparatus mayfor example also be used in medical applications where is can be used todistinguish between healthy and non-healthy tissue.

In case the above methods and apparatus are applied in the field ofbaggage inspection, a false alarm rate can be reduced significantly andby this a higher degree of automization can be achieved.

1. An inspection system for detecting a specific material of interest in an item of baggage, the inspection system comprising: a first scanner system for determining an attenuation coefficient of the item of baggage; a second scanner system for determining a diffraction pattern of the item of baggage; and a calculation unit connected to the first and second scanner systems for identifying a presence of the specific material of interest in the item of baggage on the basis of the attenuation coefficient and the diffraction pattern, wherein the first and second scanner systems are arranged at a distance from each other, wherein the first scanner system is a computed tomograph configured to apply a fan-shaped or cone-shaped radiation beam to the item of baggage; and wherein the second scanner system is a coherent-scatter computed tomograph configured to apply a fan shaped radiation beam to the item of baggage.
 2. A method of inspecting an item of baggage, the method comprising the acts of: arranging a first scanner and a second scanner at a distance from each other; scanning the item of baggage at the first scanner with a first radiation beam for determining an attenuation coefficient of the item of baggage; determining whether there is a suspicious region in the item of baggage on the basis of the attenuation coefficient; scanning an area of the item of baggage including the suspicious region at the second scanner, wherein the act of scanning the area is performed with a second radiation beam for determining a diffraction pattern of the area; determining whether there is a dangerous material in the item of baggage on the basis of the diffraction pattern; and generating a signal indicative of the dangerous material, wherein the attenuation coefficient is determined with a computed tomograph where the first radiation beam is cone-shaped; and wherein the diffraction pattern is determined with a coherent-scatter computed tomograph where the second radiation beam is fan-shaped.
 3. The method of claim 2, wherein the suspicious region is determined when the attenuation coefficient corresponds to a group of materials consisting of dangerous materials and non-dangerous materials.
 4. The method of claim 2, further comprising the acts of: issuing a first alarm in case when the attenuation coefficient corresponds to a dangerous material; transporting the item of baggage to its destination in case no suspicious region or dangerous material has been determined on the basis of the attenuation coefficient; issuing a second alarm in case a dangerous material has been identified on the basis of the diffraction pattern; and issuing a third alarm in case the diffraction pattern cannot be matched to a known material.
 5. The method of claim 2, further comprising the acts of: determining that the suspicious region is harmless in case the diffraction pattern corresponds to non-dangerous material; and transporting the item of baggage to its destination.
 6. A computer readable medium comprising a computer program wherein, when the computer program is executed on an inspection system for inspecting items of baggage, the inspection system performs the acts of: scanning the item of baggage at a first scanner stage with a first radiation beam for determining an attenuation coefficient of the item of baggage; determining whether there is a suspicious region in the item of baggage on the basis of the attenuation coefficient; scanning an area of the item of baggage including the suspicious region at a second scanner stage, wherein the act of scanning the area in performed with a second radiation beam for determining a diffraction pattern of the area, and wherein the first and second scanner stages are arrange at a distance from each other; determining whether there is a dangerous material in the item of baggage on the basis of the diffraction pattern; and generating a signal indicative of the dangerous material, wherein the attenuation coefficient is determined with a computed tomograph where the first radiation beam is cone-shaped; and wherein the diffraction pattern is determined with a coherent-scatter computed tomograph where the second radiation beam is fan-shaped. 